1. Technical Field of the Invention
The present invention relates to an active noise control system for reducing undesirable noise by producing noise canceling waves which are shifted 180 degrees in phase with respect to the noise. More particularly, the invention relates to an active noise control system suitable for reducing undesirable road noise of a frequency of 100 Hz or lower, which is generated inside the cabin of a vehicle caused by shocks or vibrations during the drive of the vehicle.
2. Description of Related Art
Known active noise control system for reducing road noise of a vehicle involves deriving a signal indicative of noise by a noise detector such as a microphone, and converting and amplifying the input signal for producing noise canceling waves from an electrical acoustic converter such as a speaker.
FIG. 8 shows one example of a frequency characteristic of road noise produced during the drive of a vehicle on a normal road. It has been ascertained that the peak in the vicinity of 40 Hz causes most unpleasant, depressing noise. FIG. 8 shows that high-level noises are also produced under the frequency of 30 Hz, but such does not present an audial problem as mentioned above because of the low frequency. However, these low-frequency components input to an electrical acoustic converter, which is generally a dynamic speaker, increase the amplitude of output signal, causing a distortional noise to be produced from the speaker.
To solve such problem, the speaker must have high performance to be able to produce large canceling waves corresponding to noise of low frequency having a large amplitude. This is, however, not practical in noise control applications in a vehicle, due to high cost and the physical size restrictions on the speaker.